Radiation causes two distinct histological changes in the mouse lung which occur at clearly separated times; radiation pneumonitis at 4-5 months after treatment (early damage) and fibrosis at 9 months post treatment (late damage). It is possible to dissociate these 2 types of damage, i.e., cause little early reaction while inducing a severe late reaction. This suggests that 2 different cell types or processes might be responsible for the 2 types of damage. It is the purpose of this study to define the conditions under which early and late damage can be dissociated, to define the radiobiological characteristics of the 2 types of damage, and define those cells which are involved in each type of damage. Answers to these questions may enable us to reduce late damage in the lung.